Recent developments in magnetic resonance tomography (MRT) and magnetic resonance spectroscopy (NMR) with polarized noble gases anticipate applications in medicine, physics and materials science. Polarization of noble gas nuclei may be achieved through optical pumping using alkali atoms, as described in the publication Happer, et al., Phys. Rev. A, 29, 3092 (1984).
The notion of optical pumping comprises the method developed by Kastler of significantly increasing by incident light radiation in material the occupation numbers of certain energy states relative to the state of equilibrium. By using optical pumping, the relative occupation numbers of the energy levels in atoms, ions, molecules and solid substances may be changed and ordering states induced. The occupation density of the optically pumped state differs noticeably from its thermal occupation probability according to the Boltzmann distribution. Through optical pumping of Zeeman levels, e.g parallel positioning of the magnetic moments of atoms and atomic nuclei may be obtained.
Typically, rubidium (an alkali atom) is used in practical operation in the presence of a noble gas and nitrogen making it possible to obtain a nuclear spin polarization of e.g 129Xe of about 20 percent. Such a nuclear spin polarization is about 100,000 greater than the equilibrium polarization in clinical magnet resonance tomographs at 1 T and 300 K. The drastic increase of the signal-to-noise ratio explains why new application areas in medicine, science and technology can be expected in the future.
Polarization refers to the degree of orientation (ordering) of the spins of atomic nuclei or electrons. For instance, 100 percent polarization means that all nuclei or electrons are oriented likewise. Polarization of nuclei or electrons is tied to a magnetic moment.
Polarized xenon is, for instance, inhaled by or injected in a human being. 10 to 15 seconds later, the polarized xenon accumulates in the brain. The distribution of the noble gas in the brain is determined by using magnetic resonance tomography. The result is used for further analyses.
The choice of an noble gas depends in each case on its actual application. 129Xe exhibits great chemical shift. If, for instance, xenon is adsorbed on a surface, its resonance frequency changes significantly. Moreover, xenon is soluble in lipophilic liquids. When such properties are desired, xenon is applied.
The noble gas helium is hardly soluble in liquids. The isotope 3He is therefore regularly used when cavities are concerned. The human lung is an example of such a cavity.
Some noble gases have useful properties other that those mentioned above. For instance, the isotope 83Kr, 21Ne and 131Xe have a quadrupole moment, of interest, e.g for experiments in basic research or in surface physics. However, these noble gases are very expensive, which makes them unsuitable for applications, in which greater amounts are used.
From the publication Driehuys, et al (Appl. Phys. Lett. (1996) 69, 1668) it is known how to polarize noble gases in a polarizer in the following way.
Based on a gas supply, a gas flow consisting of a mixture of 129Xe, 4He and N2 is enriched in a Rb container with Rb vapor and conducted through a pump cell. Using a laser, circular polarized light is produced, i.e., light in which the spin momentum or the spin of the photons exhibits the same direction. In the pump cell, the Rb atoms are optically pumped as an optically pumpable species with the laser beam (λ˜795 nm, Rb Dl line) longitudinally to a magnetic field, thereby polarizing the electron spins of the Rb atoms. The spin momentum of the photons is thus transferred to free electrons of alkali atoms. The spins of the electrons of the alkali atoms thus vary greatly from thermal equilibrium, i.e., the alkali atoms are polarized. Through the collision of an alkali atom with an noble gas atom, the polarization of the electron spins is transferred to the noble gas atom, whereby polarized noble gas forms. The polarization of the electron spins of the alkali atoms created by optical pumping of alkali atoms is thus transferred through spin exchange of the alkali electron to the nuclear spin of the noble gases, as was first shown by Bouchiat with an Rb/3He system.
From Appelt et al (S. Appelt, A. Ben-Amar Baranga, C. J. Erickson, M. V. Romalis, A. R. Young, and W. Happer, Phys. Rev. A (1998), 58, 1412) regarding the theory of two-body collisions, it is known how to produce spin exchange between a pair of alkali-metal atoms.
From WO 99/08766 (U.S. Pat. No. 6,318,092), it is known how to apply, besides an initial optically pumpable alkali metal, an auxiliary alkali metal as a non-optically pumpable species. The optically pumpable species thereby transfers the electron-spin polarization to the non-optically pumpable species, whereby effectively an increase of the polarization degree of the noble gas occurs.
Alkali atoms are used, since they possess a large optical moment of dipole, which interacts with light. Furthermore, each alkali atom exhibits a free electron, so that unfavorable interactions between two or more electrons per atom cannot occur.
Cesium would also be a well-suitable alkali atom, which is superior when compared to rubidium for obtaining the above-mentioned effect. However, there are no lasers currently available with a sufficiently high capacity, as is needed for the polarization of xenon by using cesium.
To use a maximum possible amount of photons, when employing broad-band high-performance semiconductor lasers, optical pumping of noble gases is done at pressures of several atmospheres. Optical pumping of alkali-metal atoms thereby differs according to the type of noble gas to be polarized.
In order to polarize 129Xe, a gas mixture under a pressure of about 7 to 10 Bar is guided through a cylindrical glass cell continuously or semi-continuously. The gas mixture consists to 94 percent of 4He, to 5 percent of nitrogen and to 1 percent of xenon. The typical flow rate of the gas mixture is 1 cm per second.
In case of polarization of 3He, the required pressure in the polarizer is created by 3He itself, since the electron-spin relaxation rate of Rb-3He collisions is low. For Rb—129Xe spin exchange pumps, this is not the case, which is why the pressure is created by an additional buffer gas such as 4He. The various relaxation and spin-exchange rates cause various requirements of the polarizers.
For 3He, the nuclear-spin polarization formation times are thus on the magnitude of hours. Since, however, the rubidium-spin destruction rate for rubidium 3He collisions is relatively low, operation at high 3He pressures (>5 bar) is possible.
In contrast, for 129Xe, the nuclear-spin polarization formation times based on the greater spin-exchange effective cross section are situated between 20 and 40 seconds. Based on the very large rubidium-electron spin-relaxation rate for rubidium-xenon collisions, the xenon partial pressure should only be less than 100 mbar for optical-spin-exchange pumps in order for a sufficiently high rubidium polarization to be maintained. That is why in such polarizers 4He is employed as buffer gas in order to achieve line broadening.
The polarizers may be designed as flow polarizers, e.g for polarizing 129Xe or as polarizers with a sealed optical pumping cell, e.g for 3He.
In a flow-through flow polarizer, the gas mixture initially flows through a vessel, referred to in the following as a “supply vessel,” containing a certain amount of Rb. The supply vessel with the rubidium contained therein is heated together with the adjacent glass cell to about 100 to 150 degrees Celsius. By providing these temperatures, the rubidium vaporizes. The concentration of the vaporized rubidium atoms in the gas phase is determined by the temperature in the supply vessel. The gas flow carries the vaporized rubidium atoms from the supply vessel, e.g into a cylindrical optical pumping cell. A high-performance laser in continuous operation delivering circular polarized light with a capacity of about 100 Watt penetrates by radiation the optical pumping cell axially, i.e., in the direction of flow and optically pumps the rubidium atoms in a high-polarized state. The wavelength of the laser should thereby be adjusted to the optical absorption line of the rubidium atom (Dl line).
In other words, in order to optimally transfer the polarization of light to an alkali atom, the frequency of the light must match the resonance frequency of the optical transition.
The optical pumping cell is located in a static magnetic field BD of about 10 Gauss created by coils, especially a so-called Helmholtz coil pair. The direction of the magnetic field runs parallel to the cylinder axis of the optical pumping cell or parallel to the direction of the laser beam, the magnetic field serving to guide the polarized atom. The rubidium atoms being optically highly polarized due to the light of the laser collide in the glass cell with the xenon atoms, among others, and release their polarization to the xenon-atoms.
At the exit of the optical pumping cell, rubidium deposits on the wall due to the high melting point compared to the melting points of the other gases. The polarized xenon or the residual gas mixture is passed along from the optical pumping cell and into a freezing unit consisting of a glass flask, whose end is submerged in liquid nitrogen. The glass flask is furthermore located in a magnetic field with a strength exceeding 1000 gauss. The highly polarized xenon gas deposits on the inner glass wall of the freezing unit as ice.
At the outlet of the cooling unit, the remaining gas (4He and N2) is generally guided via a needle valve and finally released. The flow rate of the whole device may be controlled via the needle valve and a with measuring device.
If the flow rate increases too much, then there is no time for transferring the polarization from the rubidium atoms to the xenon atoms. Thus only minor polarization is obtained. If the flow rate is too low, then too much time will lapse before the desired amount of highly polarized xenon is frozen. Due to relaxation in the Xe ice, the polarization of the xenon atoms hence decreases. The relaxation of the xenon atoms is greatly delayed due the freezing, and also a strong magnetic field, which the cooling unit is exposed to. That is why it is necessary, following polarization, to freeze the noble gas xenon as quickly as possible and without loss. Relaxation through freezing cannot be entirely avoided,
However, at 77 K, one to two hours remain before the xenon polarization has decreased enough so that further application of the initially highly polarized gas no longer is possible.
A polarizer of the above-mentioned type always exhibits junctions. Junctions occur where at least two lines, through which polarized gas is guided, are connected with one another, whereby the lines usually consist of glass. The connection is established by a connecting element, such as flanges.
To polarize a single free alkali atom requires a certain energy. The required energy equals the resonance frequency for increasing the free electron of the alkali atom from a ground state to an excited state. In order to transfer the energy from a laser to the alkali atom in an optimal fashion, the frequency of the laser light should be adjusted to the resonance frequency of the alkali atom. Some lasers emit their light within a certain frequency spectrum. We are therefore not concerned with a single frequency, but a distribution of frequencies. The available laser spectrum is characterized by the so-called line width. In order to polarize alkali atoms commercially, broad-band semiconductor lasers are provided, whose frequency and line width are adjusted to the resonance frequency or the optical line width of the alkali atom.
In order to better transfer the energy from a laser to alkali atoms, collision partners for the alkali atoms are provided during polarization. As collision partners serve especially the 4He atoms. Due to the interaction or collisions with the helium atoms, the optical line width of an alkali atoms expands. Increasing the width of this atomic spectrum makes it possible to use spectrally wide and therefore economical lasers.
The number of collisions between an alkali atom and a collision partner such as *He increases with increasing pressure. For 4He, for example, the expansion of the optical line width of the alkali atom is proportional to the pressure of the helium gas. In addition, *He has the value characteristic of only affecting slightly the polarization of the alkali atoms. For the polarization of 129Xe, a gas mixture consisting to 94 Percent of 4He and with a pressure of about 10 bar is usually employed.
The laser known from prior art with a power of 100 Watt for the hyperpolarization of the Rb electrons concerns a glass-fiber-coupled diode laser with a typical spectral width of 2 to 4 nanometers. With a gas pressure of 10 bar, the line width of the optical transition of rubidium atoms is expanded to about 0.3 nanometers. In the present rubidium-xenon polarizers, in which high-performance diode lasers with a typically 2-nanometer line width are applied for optical pumping, only a fraction of the laser light is therefore utilized.
The partial pressures of 4He are up to 10 bar in the gas mixture. This is very high compared with the other partial pressures (xenon or nitrogen), and is to ensure that polarized alkali metal or noble-gas atoms rarely reach the inner wall of the glass cell and lose their polarization there, e.g through interaction with paramagnetic centers. With increasing partial pressure of 4He, the probability that the polarized atoms collide disadvantageously with the inner wall of the cell decreases.
A polarized alkali atom, such as rubidium, is able to produce fluorescence radiation. If such radiation is captured by a further polarized alkali atom, depolarization of the alkali atom occurs. The nitrogen applied for the polarization of noble gases in the gas mixture serves to capture the fluorescence radiation in order to reduce the above-mentioned undesired depolarization. The nitrogen element in the gas mixture exhibits only a small partial pressure, as does similarly xenon. This partial pressure is typically about 0.1 bar.
The heavy noble-gas atoms, e.g xenon atoms, cause strong relaxation of the polarization of the alkali atoms when colliding with the alkali atoms. In order to maintain the polarization of the alkali atoms as high as possible during optical pumping, the partial pressure of the xenon gas in the gas mixture must be correspondingly small. Even with a xenon partial pressure in the gas mixture of 0.1 bar, laser capacities of around 100 Watt are required in order to obtain a polarization of the alkali atoms of about 70 percent in the whole test volume.
In prior art, optical pumping cells of glass blown from one piece are employed. This means that the windows, through which the laser light enters and exits, is always curved or rounded. During entry and exit of the laser light, undesirable and disadvantageous lens effects occur. The laser light is focused or widened, whereby the degree of polarization deteriorates considerably. The effective cross section of the optical pumping cell is therefore not uniformly illuminated by the laser light.
A gas volume with suitable composition is compressed according to prior art by a cylindrical optical pumping cell. The laser light producing the polarization is absorbed in the optical pumping cell. The pump beam thereby radiates through the optical pumping cell in the direction of flow of the mixture comprising the optically pumpable species and the atomic nucleus to be hyperpolarized parallel to the magnetic field.
U.S. patent 2002/0,107,439 A1 discloses how laser light is radiated into a optical pumping cell against the current of a flowing mixture.
As a disadvantage, all previously known prior-art methods and devices for hyperpolarization provide only a comparatively low degree of polarization of the nuclear spins, at a maximum about 40%. The reason for this is interactions in the form of collisions of the alkali metal or noble gas against the inner walls of the optical pumping cell.